The proposed research is designed to analyze nucleo-cytoplasmic interactions which play regulatory roles in cell differentiation. The pathway of differentiation to be examined is skeletal myogenesis, and the approaches to be used are those of somatic cell hydridization and cell reconstitution. Hybrid cells constructed by fusion of chicken and rat, or mouse and rat, components will be challenged to undergo proliferation or differentiation. Myogenic expression will be assessed with species-specific antibodies directed against characteristic muscle proteins. Both newly constructed hybrid cells and clones derived from them will be examined in efforts to explore both the short- and long-term consequences of such interactions. The questions underlying the investigation concern the relative roles played by the nucleus and the cytoplasma during the developmental progression of a cell from an embryonic precursor stage to a differentiated cell of a specific type.